NCEDC Blog

The NCEDC has upgraded its mass storage system for seismic and geophysical data sets to a 2.5 TeraByte storage system.

With funding from the USGS and BSL, the NCEDC purchased in mid-1998 a DISC jukebox which with capacity for 500 magneto optical (MO) platters, a small AIT tape jukebox, and the SAM-FS Hierarchical Storage Management (HSM) filesytem software. The initial DISC configuration housed two 2.6 GByte MO disk drives, and was upgraded in spring 1999 to four 5.2 GByte MO drives and 5.2 GByte media. The DISC jukebox in its current configuration can store up to 2.5 TBytes of data.

Data stored at the NCEDC is automatically written to both the MO disk platters for near-online storage and to the 25 GByte AIT tape system. The AIT tapes are removed from the jukebox when full and stored at an offsite facility.

We recently discovered that we loaded 33,933 grm files onto the NCEDC in the WRONG BYTE ORDER. These events were inadvertently stored in VAX byte order instead of Sun byte order. The list of eventids can be found in the file

~dcmgr/byteswap.error/cuspid.16bit.log

The grm file have now been renamed with a ".unknown" suffix on the jukebox until we can correct this error. In the meantime, if you have downloaded any of these events, the data will be in the wrong byteorder. We apologize for this SNAFU.

On February 1, 1999 the NCEDC completed its move into the newly renovated space in McCone Hall along with the Berkeley Seismological Laboratory (BSL) on the UC Berkeley campus. This move brings all of the NCEDC operations back to a single site.

During the previous one and a half years, a portion of the BSL and NCEDC operations were relocated to temporary quarters elsewhere on of campus in order to facilitate the extensive seismic retrofit and programatic renovations of McCone Hall. In particular, the newly-acquired mass storage system was located in the temporary quarters.

The new facilities in the BSL provide a unified computer server room, UPS and emergency generator power for computers, networking equipment, and air conditioning, and 100 Mbit switched networking amoung the NCEDC computers, BSL computers, and 100 Mbit network connection to the core campus routers which provide the Internet connection to the NCEDC.

If your computer uses the Domain Name System (DNS) to look up the IP address of a hostname, you do not need to make any changes. Your computer will automatically find the new address once the address change has been made.

However, if your computer uses static host tables (/etc/hosts on most Unix systems), you will need to update the address in the hosts file at that time.

If you have any problems connecting to the NCEDC computer after Monday, June 16 1997, please contact Doug Neuhauser at doug@seismo.berkeley.edu.

On April 29, 1997, the USGS Northern California Seismic Network (NCSN) revised the format and relocated the entire earthquake catalog at the Northern California Earthquake Data Center. We undertook the catalog revision for the following reasons:

Revision of station names to universal format

Completion of GPS survey of NCSN site locations

Removal of phase information with suspect coda durations

Assignment of new event identification numbers

Creation of new or adjustment of existing station traveltime

Because we did not change the location procedure or velocity models, the new locations and magnitudes are nearly identical to the old locations. Minor changes result from the new station coordinates and traveltime corrections. An analysis of changes in horizontal and vertical location, origin time, rms, and magnitude for the more than 200,000 earthquakes occurring in the sample period 1980-1989 (Figure 1) shows the median changes are nearly zero. Information on the change in location or magnitude for any earthquake is available upon request from Lind Gee, lgee@usgs.gov.

The following discussion provides more detail on why we made the changes to the catalog and provides supporting documentation. For more information on the NCSN network and processing procedures, see Oppenheimer et. al. (1992).

Universal station format

A new station name format was implemented because the old format did not uniquely define a station name and component. Consequently, it was difficult to combine phase information from other seismic networks with NCSN data and fully describe the type of instrumentation deployed by the NCSN. The old format designated a station by a 3-letter name, a one-letter seismic network code, and a 1-letter component code. The Universal code designates a station and component with a combination of a 5-letter site name, a 2-letter network code, and a 3-letter component code. The new phase format based on the Universal format is quite similar to the old format, and most users are unlikely to be affected by this change. A complete description of the NCSN format is available.

GPS survey of station coordinates
In April 1994 the NCSN began a systematic effort to survey all sites in the network using GPS. The effort was an adjunct to normal site maintenance visits, so the effort took more than 3 years to complete. The datum is WGS-84, versus the 1927 North American datum previously used. Uncertainty in the GPS coordinates is approximately 3 m. The mean horizontal shift is about 100 meters, though there were several sites that were mislocated by more than 500 meters (Table 1). However, the shift in datum produces a change in epicentral position of 84 - 101 m to the west and an elevation change of -15 to -20 m. Greater shifts are likely due to incorrect determination of the original site coordinates.

We obtained GPS coordinates only for NCSN sites operating since 1994. We do not anticipate obtaining GPS coordinates for discontinued sites and portable installations for previous special studies. Site coordinates are available.

Sites with GPS coordinates can be recognized in this file as those sites for which the "latitude/longitude minutes" filed has a precision of 4 significant digits to the right of the decimal place. As shown in Figure 1, the GPS coordinates do not greatly change the earthquake locations.

Figure 1. Histograms of earthquake location differences (old - new). Median values are shown above each plot. dt is the difference in origin time in seconds. dx is the absolute difference in epicentral (horizontal) position in km. dz is the absolute difference in vertical position in km. dm is the difference in magnitude drms is the difference in solution rms in seconds (positive values indicate improvement in solution fit).

Suspect coda durations
The NCSN computes a duration magnitude (Md) for almost every earthquake in the catalog. We discovered that an algorithm in one of our processing systems computed coda durations for seismograms in which there were no P or S arrival times or any amplitude observations. We consider it unlikely that a seismogram would have a reliable coda duration if the arrival time information was unreliable. Further examination showed that the suspect coda durations from these seismograms were inconsistent with durations from seismograms that had traveltime observations. In particular, the suspect durations tended to cause the computed (Md) to be too large. Consequently, we removed these suspect readings from the catalog. Their removal affected only the magnitude of a small number of earthquakes.

Assignment of new event identification numbers

Event identification numbers were re-assigned to the class of 218,026 earthquakes without associated digital seismograms. This class of data includes all earthquakes in the catalog with origin times before 1984, earthquakes from other seismic networks, and earthquakes from the NCSN Real-Time-Picker earthquake system that operated from 9/81 to 3/97. Event identification numbers of earthquakes processed by the CUSP system remain unchanged. These new numbers are negative and decrease (i.e., increase in the negative direction) with increasing origin time. A mapping of old-to-new event identification numbers is available upon request from Lind Gee at lgee@usgs.gov.

Station traveltime corrections
As a result of the new site locations, we derived traveltime corrections for all stations in the NCSN catalog. To derive these corrections, we first relocated the entire catalog with the GPS station coordinates. We used the same location procedure, velocity models, and station traveltime corrections described in Oppenheimer et. al. (1992). We then adjusted the existing station traveltime correction with the mean traveltime residual for each station. If no correction existed for a station, the mean residual became the new station correction. The latter situation most often occurred for stations installed subsequent to the time when the velocity model was developed. We then relocated the catalog again with the new traveltime corrections. The new traveltime corrections are available from Lind Gee at lgee@usgs.gov.